1) Field of the Invention
The present invention relates to a technology for electrical-discharge surface treatment using a molded powder obtained by molding a metallic powder or a metallic compound powder as an electrode, and a pulse-like electrical discharge caused between the electrode and a workpiece.
2) Description of the Related Art
In a surface treatment technology for forming a coat on a workpiece through a pulse-like electrical discharge from an electrode, which is a green compact obtained by compression-molding a powder material, a technology for forming, at temperature close to the normal temperature, a thin hard coat containing ceramics as a main constituent is established (see, for example, International Publication Number WO 99/58744).
In the technique disclosed in the above literature, a hard ceramic coat is formed on a surface of a workpiece by controlling supply of an electrode material by electrical discharge while keeping an electrode hard to some extent and melting the supplied material sufficiently. However, thickness of a coat, which can be formed by the method, is limited to as thin as about 10 micrometers.
Examples of a technology for forming a thick film through the electrical-discharge surface treatment include a technology for forming a coat containing carbide as a main constituent on a surface of aluminum (see, for example, Japanese Patent Application Laid-Open No. H7-70761), a technology for forming a coat containing carbide as a main constituent (see, for example, Japanese Patent Application Laid-Open No. H7-197275), and a technology for forming a thick film having thickness of about 100 micrometers by extending an electrical-discharge pulse width to about 32 microseconds (see, for example, Japanese Patent Application Laid-Open No. H11-827).
However, in all the technologies disclosed in the above literatures, although a thick film is formed, the main constituent of the thick film is carbide. In other words, according to the above technologies, a dense thick film cannot be formed. Therefore, in the technologies disclosed in Japanese Patent Application Laid-Open No. H7-70761 and No. H7-197275, it is necessary to perform a remelting process using an electrode with less wear after forming a porous thick film.
For example, in the technology disclosed in Japanese Patent Application Laid-Open No. H7-197275, even when a coat that looks dense is successfully formed at first glance, the coat is porous when the coat is checked fully. In the technology disclosed in Japanese Patent Application Laid-Open No. H11-827, a thick film can also be formed when an electrode of hydride is used for forming a coat. However, the coat is dense only near a surface of a workpiece where a workpiece material and a coat material are melted. A part A of a coat formed thick as shown in FIG. 13 is porous.
In recent years, there is a demand for a dense and relatively thick coat (especially, a thick film with thickness of about 100 micrometers or more) in, for example, applications in which strength and lubricity under a high-temperature environment are required. Examples of a technology for forming a coat thick include welding for welding to deposit a material of a welding rod on a workpiece through electrical discharge between the workpiece and the welding rod (building-up welding) and thermal spraying for spraying a melted metallic material on a workpiece.
However, since both the methods require manual skilled operations, it is difficult to perform the operations on a production line, disadvantageously leading to an increase in cost. In particular, when the welding, which is a method in which heat enters a workpiece intensively, is applied to a thin material or a fragile material like a unidirectionally-solidified alloy such as a monocrystal alloy and a directionally-controlled alloy, weld crack is prone to occur, thereby disadvantageously reducing yield.
Therefore, it has been earnestly desired to establish a technology for forming a thick film having strength and lubricity under a high-temperature environment using an electrical-discharge surface-treatment technology that can perform operations on a production line while eliminating manual operations as much as possible and prevents intensive heat input in a workpiece.
The present invention has been devised in view of the circumstances and it is an object of the present invention to provide an electrical-discharge surface-treatment method for forming a dense thick film on a workpiece without using the technologies such as welding and thermal spraying.